Optical amplifiers, a term which includes the optical fibre amplifiers such as Erbium doped fibre amplifiers, fluoride doped fibre amplifiers, Erbium Ytterbium amplifiers, Raman amplifiers, Brillouin amplifiers as well as semiconductor amplifiers, solid state amplifiers and the like, are commonly employed in optical wavelength multiplexed (WDM) transmission systems as they are capable of amplifying multiple WDM channels simultaneously.
These amplifiers are typically operated in a saturated condition, that is, with an output power that is independent of the input power. The amplifier gain is thus dependent on the input power. This configuration allows each amplifier placed along the transmission path to automatically adjust its gain to compensate for losses along the line.
The amplifiers are generally designed to operate at a substantially flat spectral gain, hereafter referred to as the optimal gain, for a specified input power. However, variations within production margins and within the gain matching requirements of the system into which it is to be installed may result in the amplifier having a different optimal gain from that specified. Other factors such as temperature variations and the characteristics of other components in the amplifier may also affect the optimal gain of the amplifier. In addition to the various factors that alter the optimal gain, an amplifier installed in an optical communications system may well be constrained to operate at a non-optimal gain. Reasons for this could include variations in the actual span attenuations that the amplifier must compensate for and variations in the power levels in the traffic channels. A variations in the number of channels could also change the amplifier operating gain particularly when the amplifier does not have information about the number of channels present.
A problem with this type of amplifier operation is that the gain experienced at different input signal wavelengths depends on the population inversion, which in turn depends on the amplification. Generally, an increased population inversion will cause an increased gain at shorter wavelengths with the reverse effect occurring at reduced population inversion. In other words, a slope or tilt is present in a curve of gain against wavelength within the bandwidth of amplifier operation. Thus the relative gain between the different WDM channels depends directly on the amplification of the amplifier. These gain variations cause an imbalance in gain between channels, which leads to different signal to noise ratios at the receiver.
While this gain variation over wavelength may not be critical in a system carrying signals of a limited bandwidth, the impact is more serious for WDM systems which commonly carry 80 channels spread over 30 nm. In such a system, the difference in signal to noise ratio between the best and worst channels will be large. Since the channel manifesting the lowest signal to noise ratio sets the limit for the performance of the whole system, this implies that the system must operate well below its full capacity.
International patent application WO9836513 describes an optical amplifier that is able to amplify an optical signal without introducing a gain tilt. This is achieved by using two optical amplifiers in series, between which an attenuator is connected. The amplifiers are identically constructed and are assumed to have identical gain spectrums. The gain of each amplifier expressed in dB is assumed to be a linear combination of the gain at two known wavelengths for a fixed pump power and input power. By adjusting the attenuation of the signal power between the amplifiers, the gain of the second amplifier can be selected to apply an equal but opposite gain tilt to the input signal. A disadvantage of this known arrangement is that while the two amplifiers may be designed to have identical spectral gains, the value obtained in production may vary with the result that an unknown gain tilt will be introduced between traffic channels. While the system as a whole may tolerate a small gain tilt between channels, the accumulated gain tilt generated by cascaded amplifiers is likely to introduce unacceptable variations in the SNR of different channels. Furthermore, such an amplifier arrangement is relatively costly and requires the measurement of the pump power of the amplifiers in addition to the input and output powers.
It is an object of the present invention to provide an optical amplifier arrangement and method for controlling the same which overcomes the problems associated with prior art apparatus.
It is a further object of the present invention to provide an optical amplifier arrangement and method for controlling the same that enables gain tilt to be reliably controlled over an optical transmission path.
It is another object of the present invention to provide an optical amplifier arrangement and method for controlling the same that enables gain tilt to be controlled at relatively low cost.